The bottom line is that the ultimate determination of a vaccine's merits comes from controlled tests conducted under field conditions similar to those of your production setting. But, these studies are difficult to conduct and have been conducted for only a few vaccines. Evaluating a vaccine's effectiveness is very difficult otherwise because so many management factors can overwhelm a vaccine's effect.

Viral or bacterial vaccines may be killed or live. Live vaccines contain bacteria or a virus that has been modified (MLV). This means they've lost their disease-causing ability (attenuated) or are administered by a route that prevents them from causing clinical disease. Killed vaccines are just what the name says - they've been attenuated though a process resulting in their death.

The quality of the immune response that is stimulated can be different in ways that are currently thought to provide better protection. The details of these differences, however, are too complex to be presented here;

Less chance of allergic reactions; and,

The bacteria or virus may look and behave more like the disease-causing form of the organism.

Meanwhile, some of the advantages of killed vaccines are:

More stable on storage;

Unlikely to contain contaminating pathogens; and,

Unlikely to cause disease due to residual disease-causing characteristics.

It's A Complex Issue

Often, in our attempts to explain the MLV versus killed issue in simple terms, we not only oversimplify the issue,[6] but we make the discussion more black and white than it really is. Still more confusion can result when that desire for a simple explanation is combined with manufacturers' desires to capture market share.

With any vaccine[7], the trick is to have the attenuated organisms mimic their disease-causing cousins closely enough that the calf's active immune system will be ready to recognize the disease-causing pathogen. Then, when infection occurs, it either will be interrupted before disease results, or the severity of the resulting disease will be reduced.

Note that vaccines can't prevent infection. The offending pathogen must get inside the body to come under fire from the vaccine-stimulated active immune system.

Infection is prevented by a different part of the immune system called the innate immune system. For example, bacteria that cause pneumonia must first overcome the mucous and cilia lining the upper airways.

Then they must get past defense cells in the lower airways, and finally penetrate the respiratory tract membranes. If the bacteria are unable to accomplish all this, infection is prevented and vaccine-stimulated immunity is a moot issue.

These partitions of the immune system are purely arbitrary. But, they are necessary to explain and discuss this incredibly complex system. Because they're arbitrary, there can be overlap and confusion.

In general, vaccines do not affect the innate immune system. The innate immune system is more impacted by our management and husbandry practices[7].

This is a complex issue partially because affecting the outcome of infection is not just about vaccines and the calf's immune system. It also involves characteristics of the offending pathogen - where and how the bugs try to hide from the immune system and how they cause disease.

Some pathogens have the ability to hide out inside the animal's cells. A vaccine that does not stimulate the part of the active immune system that is able to recognize infected cells, may not be able to affect such a pathogen and disease will result. An example is the bacteria causing brucellosis.

Conversely, a vaccine can effectively stimulate the part of the active immune system that recognizes infected cells, but do a poor job of stimulating the production of the proteins (antibodies) that neutralize pathogen-produced toxins in the blood. This situation may not provide good protection against a disease such as tetanus, in which disease is caused by toxins that tetanus-causing bacteria dump into the bloodstream.

We will never be able to say that using a killed or a MLV vaccine is always superior. The answer will depend on the targeted pathogen, as well as the nature of the relationship between the calf, pathogen and vaccine. Specific vaccine recommendations should be made by a veterinarian familiar with your operation, your type of cattle and the disease problems they typically experience.

Clearly, the vaccines we employ today enjoy a high level of safety. And, the progress continues. Today, studies in areas such as injecting animals with key portions of naked DNA from a pathogen or inserting DNA from pathogens into plants are being explored as means to enhance vaccine safety and efficacy.